It is generally recognized that increased production of cultured fish is necessary to meet the demand for high quality protein food in both the United States and the rest of the world. This results in high intensity culture of fish with its related problems of handling, diet, water quality and disease control. As density and growth rates are increased, disease problems increase and in many cases become the limiting factor on production. The concept of disease prevention, rather than attempted cure, has always been regarded as the most rewarding approach to control. It has been recognized that immunizing agents can be used as a complement to other methods of disease control. A problem exists in providing economic methods of mass vaccination. In the past, vaccines for fish have been delivered by several methods including parenteral injection; oral administration by incorporating the vaccine in the fish's diet; hyperosmotic inflitration by placing the fish in a hyperozmotic solution followed by a vaccine bath; direct immersion in vaccine suspension; or, by direct addition of vaccine to water in which fish are held. The report entitled "Spray Vaccination: A Method for the Immunization of Fish" by R. W. Gould and others, Fish Pathology XIII (1), 63-68, 1978.6, comments that each of these techniques has its inherent advantages and disadvantages. Although intraperitoneal injection appears to be most effective, this method is time consuming and stresses the fish being vaccinated. Oral administration is a desirble method of vaccine delivery, but in some cases has not provided high levels of resistance. Hyperosmotic infiltration and direct immersion may be useful in vaccinating small fish but may not be economical for larger fish. The Fish Pathology article further comments that the addition of vaccine to water has only had limited experimental use. The Fish Pathology article states that another method for mass immunization of fish is administration by spraying fish with antigens. The Fish Pathology article indicates that fish were spray vaccinated at pressures from 6.3 to 7.0 kg/cm.sup.2 utilizing a commercial sandblasting gun with a venturi-feeding reservoir containing bacterin and explains that experimental groups of fish were placed in a dip net and sprayed five to ten seconds with the tip of the spray apparatus positioned 20 to 25 cm from the fish. The fish were moved in the net so that each received direct application of bacterin on some portion of the body surface. An atomizer was also used in a similar manner to spray vaccinate at zero pressure.
Improvement is needed in the apparatus used in spray immunizing fish. The present invention provides an apparatus which is particularly adapted to facilitate the mass immunization of fish using a low pressure shower.